Use of a physiologically-based pharmacokinetic model to explore the potential disparity in nicotine disposition between adult and adolescent nonhuman primates

Highlights

• A PBPK model was developed for nicotine in adult and juvenile monkeys

• Juvenile monkeys clear nicotine and cotinine faster than adults

• Age-dependent differences in monkeys may help inform nicotine kinetics in humans

Abstract

The widespread use and high abuse liability of tobacco products has received considerable public health attention, in particular for youth, who are vulnerable to nicotine addiction. In this study, adult and adolescent squirrel monkeys were used to evaluate age-related metabolism and pharmacokinetics of nicotine after intravenous administration. A physiologically-based pharmacokinetic (PBPK) model was created to characterize the pharmacokinetic behaviors of nicotine and its metabolites, cotinine, trans-3′-hydroxycotinine (3′-OH cotinine), and trans-3′-hydroxycotinine glucuronide (3′-OH cotinine glucuronide) for both adult and adolescent squirrel monkeys. The PBPK nicotine model was first calibrated for adult squirrel monkeys utilizing in vitro nicotine metabolic data, plasma concentration-time profiles and cumulative urinary excretion data for nicotine and metabolites. Further model refinement was conducted when the calibrated adult model was scaled to the adolescents, because adolescents appeared to clear nicotine and cotinine more rapidly relative to adults. More specifically, the resultant model parameters representing systemic clearance of nicotine and cotinine for adolescent monkeys were approximately two- to three-fold of the adult values on a per body weight basis. The nonhuman primate PBPK model in general captured experimental observations that were used for both model calibration and evaluation, with acceptable performance metrics for precision and bias. The model also identified differences in nicotine pharmacokinetics between adolescent and adult nonhuman primates which might also be present in humans.

Introduction

Nicotine (C₁₀H₁₄N₂, CAS number 54-11-5, Molecular weight 162.23), though not the direct cause for most tobacco related diseases, is responsible for the addictive effects of tobacco products. Tobacco use remains the leading cause of preventable disease and deaths per year in the United States (FDA, 2017). Of particular concern are youth, in light of the potential increased risk for addiction and health issues. Both animal studies using adolescent (postnatal day, PND25) and adult (PND85) male Sprague-Dawley rats (Gellner et al., 2016), early adolescent (PND28), late adolescent (PND38), and adult (PND90) male Sprague-Dawley rats (Belluzzi et al., 2004), as well as clinical data, indicate that adolescents display greater sensitivity to the reinforcing properties of nicotine and are more likely to become nicotine dependent compared to adults (Chen and Millar, 1998). Tobacco use is primarily initiated and established during adolescence.

Nicotine pharmacokinetics plays a critical role in nicotine addiction and nicotine replacement therapy (Le Houezec, 2003). Pharmacokinetic profiles of nicotine have been widely studied in adult humans. Given that nicotine is a weak base with a pKa of 8.0, absorption of nicotine across biological membranes relies on the pH of the environments, since nicotine does not rapidly cross membranes in its ionized state. Following absorption, nicotine is largely distributed into body tissues. The plasma protein binding of nicotine is <5%. Nicotine is extensively metabolized by the liver, with the majority (70–80%) converted to cotinine (C₁₀H₁₂N₂O, CAS number 486-56-6, Molecular weight 176.22) via oxidation. In addition, nicotine is also excreted by the kidney via glomerular filtration and tubular secretion, accounting for 5% of the total clearance (Benowitz et al., 2009). The terminal elimination half-life of plasma nicotine is about 2 h in adult humans (Matta et al., 2007).

Urinary clearance rates (half-lives) of cotinine in newborns whose mothers smoked tobacco have been reported (Etzel et al., 1985; Dempsey et al., 2000). In other studies, infants and youth were exposed to second hand smoke, then placed in a smoke-free environment before urine collection, and the urinary clearance of cotinine was calculated from the collected samples (Collier et al., 1994; Leong et al., 1998). These reported urinary half-lives were quite variable and conflicting, reflecting the shortcomings in using these opportunistic methods. When deuterated cotinine was administered to youth aged 2.5 to 82.4 months, urinary half-life estimates for cotinine were measured without interference of low-level contamination. The results showed that mean half-lives of cotinine ranged from a low value of 13.7 h in infants under one year of age and 19.6 h for youth 5–6 years of age (creatinine uncorrected) (Dempsey et al., 2013). However, to the best of our knowledge, no plasma pharmacokinetic studies of nicotine in youth are available in the literature and the difference in the pharmacokinetic characteristics of nicotine and its metabolites between youth and adults remains unclear.

Among different experimental animal models, nicotine metabolism in nonhuman primates is more comparable to humans (Matta et al., 2007). Like humans, nicotine is mostly converted to cotinine via oxidation, referred to as C-oxidation, in nonhuman primates, as identified in African green monkeys (Vervets, Cercopithecus aethiops), with CYP2A6 (referred to as CYP2A6agm) as the primary enzyme and a minor contribution from CYP2B6 (referred to as CYP2B6agm) (Schoedel et al., 2003). CYP2A6 and CYP2B6 expression, substrate specificity, and mediation are similar between nonhuman primates and their human orthologues (Schoedel et al., 2003), making nonhuman primates a more appropriate model than other animal models for the investigation of human nicotine pharmacokinetics. In addition, similar to humans, approximately 80% of nicotine is metabolized to cotinine in nonhuman primates (Poole and Urwin, 1976). Comparable terminal elimination half-lives for nicotine and cotinine have been observed between nonhuman primates and humans (Hukkanen et al., 2005). The terminal elimination half-life for nicotine is 1.6 h for macaque monkeys comparable to 2 h for adult humans (Hukkanen et al., 2005). For cotinine, a primary metabolite that is either cleared in urine or metabolized in the liver, the terminal elimination half-life is 9.2 h in the macaque monkey and 13.3 h in adult humans (Seaton et al., 1991).

In the current study, a nonhuman primate squirrel monkey model was used to evaluate the plasma and urinary pharmacokinetics of nicotine in adolescents and adults after intravenous administration of nicotine. The adolescent squirrel monkeys (1 to 3 years old) used in this study reflect a human youth's age of 6 to 16 years (Rowe, 1996; Beer et al., 2017), an important phase for neural development and tobacco use initiation. Further, a physiologically-based pharmacokinetic (PBPK) model was developed to characterize the plasma pharmacokinetics and urinary excretion of nicotine and its metabolites, cotinine, trans-3′-hydroxycotinine (3′-OH cotinine, C₁₀H₁₂N₂O₂, CAS number 34834-67-8, Molecular weight 192.21), and trans-3′-hydroxycotinine glucuronide (3′-OH cotinine glucuronide, C₁₆H₂₀N₂O₈, CAS number 132929-88-5, Molecular weight 368.34) for both adult and adolescent squirrel monkeys. Nicotine, as a highly addictive chemical compound, is also well known to cause adverse effects on the heart, lung, reproductive system, and kidney (Mishra et al., 2015). Nicotine's major metabolite, cotinine, which has been historically considered less harmful than nicotine, has also been shown to adversely impact the heart and reproductive system (Bastianini et al., 2018). As such, both nicotine and cotinine are included in the current model. In addition, to better track mass balance, the two major inactive metabolites, 3′-OH cotinine and 3′-OH cotinine glucuronides are also described in the model.

While several PBPK models have been developed to describe the pharmacokinetics of nicotine and cotinine in humans and experimental animal models, the existing models focused primarily on adults (Plowchalk et al., 1992, Robinson et al., 1992, Teeguarden et al., 2013, Gajewska et al., 2014, Saylor and Zhang, 2016). Additionally, to date there have been no nicotine PBPK models created for adult and adolescent nonhuman primates. Such knowledge would provide critical information to help identify age-dependent differences in nicotine pharmacokinetics in humans.